Solid-state laser systems are characterized in that they have a solid-state laser gain medium which converts energy from an optical pump source to a coherent output laser beam. The pump source can be one of many available energy-producing systems such as flash lamps or semiconductor laser diodes. The energy produced by the pump source is incident upon the laser medium and absorbed by the laser medium.
The absorbed energy in the laser medium causes the atoms in the laser medium to be excited and placed in a higher energy state. Once at this higher state, the laser medium releases its own energy which is placed into an oscillating state by the use of a laser resonator. The laser resonator includes at least two reflective surfaces located on either side of the laser medium. The laser resonator can be designed to continuously release a laser beam from the system. Alternatively, the resonator can be designed such that when the energy oscillating through the laser medium reaches a predetermined level, it is released from the system as a high-power, short-duration laser beam. The emitted light produced from the solid-state laser system is generally coherent and exits the system in a predefined area.
In many systems, the laser medium is Neodymium-doped, Yttrium-Aluminum Garnet (Nd:YAG). A laser medium made from Nd:YAG absorbs optical energy most readily when the energy is at a wavelength of approximately 808 nanometers (nm). Thus, the source to pump the Nd:YAG laser medium should be emitting light energy at approximately 808 nm. Gallium arsenide semiconductor laser diodes can be manufactured with dopants (e.g. aluminum) that will cause the emitted light to be in a variety of wavelengths, including 808 nm. Thus, the semiconductor laser diodes, which are lasers by themselves, act as the pump source for the laser medium.
The conversion of optical energy into coherent optical radiation is accompanied by the generation of heat which must be removed from the device. Cooling of the laser medium reduces the build-up of temperature gradients and, thereby, the strain and stress in the laser medium and also avoids the likelihood of laser medium fracture due to high thermo-elastic stress. Also, variation of the refractive index and its associated optical distortion can be largely controlled or avoided by effective cooling. The result is improved beam quality and/or increased average output power.
Diode array performance is also strongly dependent on temperature. Not only is the output power a function of temperature, but the wavelength of the emitted energy that is to be absorbed by the laser medium is also a function of diode temperature. To maintain desired array performance and to prevent the diode array from being destroyed by overheating, cooling of the area surrounding the array is also important.
Other laser assembly components, some having low damage thresholds, also require close temperature control. For example, beam dumps, that absorb and dissipate incident laser energy to ensure that incident laser energy will not emerge to interfere with wanted parts of the beam, produce heat. Nonlinear crystal assemblies for the conversion of wavelengths in a laser system utilize temperature control systems for the precise control of these temperature-sensitive crystals. Careful attention is also given to the optimal transfer of heat from acousto-optic Q-switches.
It has been an objective for laser manufacturers to develop high-power, solid-state systems. As the output power in these system increases, the waste heat increases which puts more demands on cooling systems and necessitates larger volumes in which to provide adequate cooling. Hence, the efficient and effective removal of waste heat from diode arrays, the laser medium, and other heat-generating components is an important factor in developing compact, high-powered laser systems.
Known laser systems utilize active cooling. Active cooling systems may use thermoelectric coolers, or fluid systems having mechanical pumps and coolant carrying tubing operated at pressure. However, active cooling systems consume additional power to control the temperature of the laser and require additional space in the laser system. Furthermore, active cooling requires feedback control systems to adjust the amount of cooling that is necessary to maintain the laser components at the appropriate temperature.